Nylon polytetrafluoroethylene composition and article



L.. L. sToTT 2,975,128

NYLON POLYIEIRAFLUOROEIHYLENE COMPOSITION AND ARTICLE March 14, 1961Original Filed Sept. 2l, 1954 JNVENTOR. ou/5 ,'5707'7' United StatesPatent O 2,975,128 NYLON POLYTETRAFLUOROETHYLENE `COMPOSITION ANDARTICLE Continuation of abandoned application Ser. No. 457,370,

Sept. `21, 1954. This application Aug. 21, 1958, Ser. No. 756,749

9 Claims. (Cl. 252-12) This invention relates to the art of molding highmolecular weight synthetic linear polyamides. The present application isa continuation of .my application Serial No. 457,370, ledSeptember 21,`1954, entitled Nylon Polytetrauoroethylene Composition and Article, andabandoned August 25, 1958. Said application Serial No. 457,370 was acontinuation-in-part of my applications Serial No. 227,283, led May 19,1951, abandoned September 24, 1954, and SerialNo. 272,966 led February23, 1952, now Patent 2,695,425, issued November 30, 1954. The syntheticpolymeric 'materials used in'the practice of this invention are thesynthetic linear polyamides of the general type described in UnitedStates Patents 2,071,250, 2,071,254 and 2,130,948. The polymers theredescribed are high molecular weight products which generally can beobtained crystalline in structure as evidenced by X-ray powderdiffraction patterns of the polymers in the massive state.

The polyamides of the present type, generally speaking, comprise thereaction product of a linear polymer-forming composition, for example,one consisting essentially of bifunctional reacting material, whichcomprises in substantial amount molecules containing two amide-forminggroups each of which is complementary to an amide-` forming group inother molecules in said composition.

These polyamides as described above, or as otherwise identie'dhereinafter, can be obtained, for example, by self-polymerization ofmonoamino-monocarboxylic acid, or by reacting a diamine with a dibasiccarboxylic acid in substantially equimolecular amounts, it being under#stood that reference herein to the amino acids, diamines,

Vanddibasic carboxylic acids, is intended to include the equlvalentamide-forming derivatives of these reactants.V

condition in comparison to other thermoplastic materials,

such as cellulose acetate and polystyrene. These charare preparedcommercially, eitherby machining solid nylon, such as nylon rod, or bymelting nylon and forming it by injection molding. Both of these methodshave certain drawbacks. Bearings machined from nylon rod, for instance,are relatively expensive to make and involve much waste. Furthermore,the manufacturing techniques for producing nylon rod, particularly forthe larger sizes, usually introduce. severe strains which must beremoved by conditioning. 'Ille injection molding technique requirescomplicatedy and expensive apparatus, high cost molds, and alsofrequently results in a product having many strains. These strains causebearings prepared by injection molding to seize readily if largeclearances are not' provided. The presently used molding techniquesdepend upon heating a vpolyamide above its melting point and` exertingpressurey on the molten material. The

strains produced in the resultant article come as a con-V sequence ofcooling the molten material and, at least in part, are due to arelatively high'volumetric Vshrinkage on solidication. n A

In accordance With my U.S.V applicationSerial No.Y 227,283, filed May19, 1951, molded nylon articles containing llers are described. Thesearticles are made by These linear polyamides includeralso polymers ob- Ytained by admixture Aof other linear polymer-forming reactants, as forinstance glycol-dibasic acid mixtures in the case of polyester-amides,with the mentioned polyamide-forming reactants, The best results in thepractice of the invention described herein, however, are

obtained with unmodified straight polyamides.- In the" interpolymers, aswell as in `the simple polyamides, the` average number of carbon atomsvseparating the amide groups is at least two. On hydrolysis.with'hydrochloric acid, the amino acid polymers yield the amino acidhydrochloride and the dibasic carboxylic acid. In any case, thepolyamides are limited lto those which are soluble in phenol at roomtemperature and are insoluble.Y in ethylene glycol except at'temperatures above about 140 C. For the sake of simplicity the linearpolyamides described above will be referred to herein as nylon.

Although these materials werejoriginally introduced nylons, such aspolyhexamethylene adipamide, and polyf hexamethylene sebacamide, arecharacteri7edV by relai tively sharpmeltingpoints andhigh uidityin the.molten p polyamides compressing nylon powder preferably having adiameter of 40 micronsgor less and a finely divided ller'with 'suf-4 iiicient pressure so that they may be handled. TheV corn'-Vnylon-polytetrauoroethylene mixture asv compared to articles-made fromnylon alone.

Itis technically extremely diflicult to get a uniform,V

dispersion of polyte'tralluoroethylene in, molten nylon or mechanicallyto mix finely divided nylon andjpo'ly-l tetrafluoroethylene particularlywhere the polytetraf, vif fluoroethyl'ene ller is desired in highconcentrations#"` The melting of nylon containing the polytetrafluoro-`l ethylene causes an uneven distribution of the filler inthe i n moltennylon, while the mechanical mixing of the ver/yi"-`r ligh-t powderednylon and polytetrafluoroethylene does i not give iinishedv products ofacceptable strength except where polytetrauoroethylene is present inamounts ofY 5 percent or less by weight. These diiiiculties of mixingVhaverlbeen -overcome'by the process herein describedv to give ahomogeneous `mixture of lnely` divided f i and polytetrailuoroethylenewhich upnfY pressing and sintering yield products of good strength-'f1 pf Y It is an object of this invention to produce powder and 1 i moldedarticles of'modied nylon, i.e., nylon having a' Y ller which impartsphysical characteristics which differv from those Vof usual nylonarticles. Another object is to prepare molded nylon articles'havinggreater dimensional" y stability during humidity changes thanunmoditiednylon articles. A further Objectis to produce modiedfnylonarticles having lower coeicients of friction than exhibitedk by purenylon articles.

tended period of time.

Nylonpowder having ultimate particle sizeuofrlesa n 2,975,128 f YPatented Mar. 14, .19161V 'i toA l. In addition certain fillers impartimportanti Such Still anotherobject is Yto 'pro-Ej? duce nylon articleshaving polytetrauoroethylene evenlyk Y distributed throughout thefinished articles. Yet'yanoth object is to produce modified nylonbearings Yandfbush=` ingsv which exhibit low coeicients of frictionoverall x 'f than 40 microns is mixed thoroughly with finely dividedpolytetrauoroethylene and compressed to the desired shape with suicientpressure so that the shaped green article may be handled. The compressedarticle isthere-` after heated to a temperature suicient to sinter theparticles of nylon together without inducing any substantial moltenphase. The article may be annealed to remove any residual strains.

It is essential that a substantial portion of the nylon particles bebelow about 40 microns in diameter and preferably below 25 microns, butthe optimum size appears to be l microns or less. Nylon of this size maybe obtained in accordance with the process described in United Statesapplication Serial No. 95,587, now U.S. Patent 2,592,616, tiled in thenames of Louis L. Stott and Laurence R. B. Hervey on May 26, 1959;United States application Serial No. 273,566, now United States Patent2,742,440 filed in theV names of Louis L. Stott and Laurence R. B.Hervey on February 26, 1952; and United States application Serial No.202,4-05, now United States Patent 2,639,278, filed in the names ofLouis L. Stott and Laurence R. B. Hervey on December 22, 1950. Thesemethods disclose that nylon may be dissolved in mixtures of loweralcohols anl water or in methanol alone under pressure and at elevatedtemperatures, and that nylon may be dissolved in polyhydric alcoholsmerely by heating them together. Oxygen is excluded during the heatingstep. Upon cooling, the nylon precipitates as a fine powder which, whenwashed and dried, is suitable for the present process. In the case ofpolyhydric alcohols cooling may conveniently be accomplished by addingwater to the hot nylon solution. If waste nylon is employed, undissolvedmaterial is preferably removed when the polymer is in solution.

It has been found preferable to use nylon which has been prepared in themanner just described. This may be because of the difliculty inobtaining mate-rial having an average ultimate particle size of lessthan 40 microns as is obtained by the process described. It is also tobe recognized that the product obtained by precipitating nylon asdescribed in the above-identied applications is crystalline in characterand becomes more so upon sintering as disclosed by X-ray diffractionpatterns. But for whatever the reason, the nylon must be reduced in sizeto the order of less `than 40 microns in diameter.

The polytetratluoroethylene used as an additive should preferably be ina finely divided state. However, the average ultimate particle size ofthe polytetratiuoroethylene depends to some extent upon the quantity ofpolytetrauoroethylene incorporated in the nylon. Thus, it has been foundpermissible to use relatively larger polytetrailuoroethylene particlesizes in polyamide polytetrailuoroethylene mixtures containing smallquantities of polytetrauoroethylene. The reason for this apparent effectintroduced by particle sizeis not known.

I have found -that contrary to established procedures for handlingthermoplastic materials it is possible to cold press thepolyamide-polytetratluoroethylene mixture, remove the article from themold and subsequently sinter the finely divided materials into finishedarticles, provided the starting particle sizes are of the orderindicated. No precautions need be observed with respect to the type ofmetals coming in contact with the nylon. Although polytetrauoroethylenemay be added to the finely divided polyamides in amounts of from 2percent up to 75 percent, by weight of the nylon-polytetrailuoroethylenemixture, the optimum amount of polytetrauoroethylene for molded articlesshowing low coefficients of friction and good wear properties dependsupon the hardness of the nely divided polyamide and its ability to bondthe filler in the finished article. Thus, for example, a hardpolyhexamethylene adipamide can contain more of the relatively softerpolytetrauoroethylene and the amount of polytetrauoroethylene may rangefrom about 30- to `75 per-A cent by weight of the mixture, while aprefered Arange is from about 40 to 50 percent. In a moderately hardpolyhexamethylene sebacamide, for example, the quantity ofpolytetrafluoroethylene may range from about 10 to 50 percentpolytetrailuoroethylene by weight of the total mixture while thepreferred range is about 20 to 30 percent. With a softer polymer such asan epsiloncaprolactam containing relatively large amounts of monomer thequantity of polytetrauoroethylene may range from 2 to 15 percent whilethe preferred range is about 3 to 10 percent by weight of total mixture.

If other fillers are added in addition to polytetrafluoroethylene, thetotal quantity of filler should not exceed percent by bulk volume of thepowder mixture.

Polytetrauoroethylene, as do other fillers, reduces the hygroscopicexpansion of the final molded articles as compared to articles made ofnylon alone. Articles molded from a nylon-polytetrafluoroethylenemixture possess another very important and unexpected advantage overarticles molded from nylon or polytetrauoroethylene powders alone. Thisadvantage consists of a marked decrease in coeicient of friction overthe entire test or use period. It has been noted that bearings andbush-` ings molded from a linely divided polyamide powder, such aspolyhexamethylene adipamide, show a decided and sharp early peak incoefficient of friction. This peak generally, but not always, occurs atabout 4 to 5 minutes after the test is begun. After this peak isattained, the coecients of frictiondrop rapidly and then level oi to amore or less constant value. Even though this peak may be of acomparatively short duration, the

period of its existence may well be sutcient to cause overheating of theshaped article and may result in seizure or excessive wear. Thereduction or elimination of this peak by the addition ofpolytetrauoroethylene, therefore, makes it possible to materially reducethe effects of the peak and to maintain coecients of friction at aminimum. In addition to reducing or entirely eliminating this peak incoeliicient of friction, the use of polytetrafluoroethylene alsomarkedly reduces the over-all, long term coeflicient of friction, thusreducing the overall wear.

For example, sample molded pieces were made up of pure nely dividedpolyamide molding powders and of mixtures containing varying quantitiesof polytetrafluoroethylene. The finely divided polyamides were preparedby dissolving 240 grams of the polyamide (either virgin or scrapmaterial) in 1500 grams of ethylene glycol in a carbon dioxideatmosphere at 193 C. Suicient quantities of finely dividedpolytetrafluoroethylene to make up the desired composition were added tothe hot solution and the solution was allowed to cool to C. when it wasquenched with an excess of cold water. The coprecipitatedpolyamide-polytetrauoroethylene mixture was then Washed to remove theethylene glycol and dried.

All'of the varying compositions made up were completely homogeneouspowder mixtures.

Portions of each of the powder mixtures prepared in the above describedmanner were pressed at room ternperature at a pressure of 25 tons persquare inch to give sample pieces suitable for use in a machine designedto measure coeicient of friction. These sample pieces were removed fromtheir molds and heated in vacuum, the ones containing polyhexamethyleneadipamide to 263 C., those containing polyhexamethylene sebacamide to220 C., and those containing the polymer of epsiloncaprolactam to 215 C.After the sample pieces were heated throughout to the sinteringtemperature, they were removed from the vacuum and cooled.

The sample pieces were then evaluated for coeicients of friction byrubbing against them a metal wear cap attached to a vertical rotatingshaft. A load was applied` by a lever-arm and weight system which thrustthe bushing against the wearing surface of the wear cap. The cap wasrotated at 620 r.p.m. and loads of 50 and 200'p.s.i. were applied.

Interms of this test, the most desirable sample is' one` which displaysa low coeticient of friction (both during" break-in and at t'ne end ofthe test), low wearing` rate, absence of wear on the Wear cap and lowtemperature of wear cap during operation. Table I illustrates the effectof the addition of polytetrafluoroethylene to 'poly-V hexamethyleneadipamide and shows that at the beginning of the tests the coefficientsof friction were as good or better than for the polyamide alone, andthat the peaks were materially reduced.. At the end of the test the coeicients of friction were markedly reduced while wear was reducedeither to a negligible'quantity or a fraction of that exhibited by thepolyamide sample. In addition to the quantitative data in Table I, aqualitative examination of the wear cap showed less burning where .thelsample contained polytetrafluoro ethylene; approximately the same degreeof heat generation for all samples; aboutV the same extent of streaks orscratches put on the wear cap; and somewhat less deposit of the samplematerial on the wear cap in the case of thepolytetrafluoroethylene-containing samples.

` TABLE I Coecz'ent of friction-friction and wear properties at 50p.s.i. and 620 r.p.m.

Table II gives similar data for polyhexamethylene adipamide andpolyhexamethylene sebacamide mixed with polytetrauoroethylene and testedunder a higher load then in Table I. The same reduction in the coecientof friction achieved by the addition of polytetrauoroethylene is shownby these data.

I TABLE Il Coefficient of friction-friction and `rear properties atr-200 p.s.. and 620r.p.m;

e Y out and still dispersed'in the swelling agent or swelling agent isremoved and the'inely divided poly` amide is still wet. In all caseswhere the polytetrauoroethylene is added in amounts exceeding 5 percentby weight of the mixture polytetrafluoroethylene -is added ,to thepolyamide when wet.

The mixture of nylon powder with polytetrauor *i v ethylene isbriquetted by' the use of sufficient pressure'to Y cause the resultantshaped article to withstand moderate shocks incident to its handling.VThe pressures employed range generally between about 10 tons per squareinch and tons per square inch. The pressures do not seem to be critical,but it has been found that about 25 tons per square inchis averyjsatisfactory pressure. vPressure in the order of 3 tons peresquareinch yields a briquette which may be handled only with considerable'careand when tired has a compressive strength of less than onehalf that of asimilar piece pressed at 25 tons per squareY inch. Pressures in excessof 75 tons per square inch are not required. The resultant coldpreformed article 'is then sintered by heating it under non-oxidizingconditions to a temperature below the melting point of the nylon presentfor a'time suicient to cause the article to be strong and hard whencooled. In accordance with ap plication Serial No. 272,966 filedFebruary 23, 1952, in the name of Louis L. Stott, I have found that ifseveral pieces are molded from nylon powder at room temperature andsintered at various temperatures ranging from slightly above roomtemperature upto nearly the melting point and the resultant pieces aretested for compression strength, that an interesting phenomenon becomesapparent. .Referring now to the drawing, the curves are plots of theloads required to break bearings 1 inch long, 1/2, inch LD. and'3/4 inchO.D. when the loads are applied alongthe surface perpendicular to theYaxis. Curve A is a plot of epsilon-caprolactam polymer sintered atvarious temperatures; curve B is a plot of the loads required to breaksimilar'bearings of polyhexa-y methylene sebacamide sintered at varioustemperatures, and curve C is a plot of the loads required tobreaksimilar bearings of polyhexamethylene adipamide at various ftemperatures. The bearingswere Vsintered inpvacuo.

Coefcient .of Friction Percent Percent Polyamide Poly- Polytetamideratluoro- 4-5 At end Wear ethylene AtStart min. 2% after Peak hrs.A2%hrs.

- '1'n.- Polyhexamcthylene A Adipamide 100 0 0.14 0.38 0.24 0.0308 Y Do40 60 0.18 0.19 0.14 Vngligr- Polyhexamethylene f Sebacamide 100 0 0.230.29 0.19 0.0625 Do 70 30 0.12 0.28 0.05 0.0633

It `will be noted at these higher loads the addition ofpolytetrauoroethylene tov polyhexamethylene adipamideV improves thewearing properties,`but'the addition ofv 30% polytetrailuoroethylene tothe lower melting point polyhexamethylene sebacamide appears todecreasethe wearing ability. However, at thelower load of 50 p rsii. theaddition of 30% polytetrafluoroethylene to polyhexa'- methylenesebacamide improves the wearing character.- istics.

To incorporate the polytetrailuoroethylene,'it may be Wet mixed with thenylon, or'may be mixed with the nylon after it has been dissolved in anagent which is a swelling agent for the nylon at elevated temperaturesand a nonswelling agent at room temperature.The-nylon-polytetrafluoroethylene mixture is then precipitated out as ahomo`` geneous mixture by cooling or Vadding la ynon-swelling agent.` Inthe last named method vof mixing, the polytetrailuoroethylene may beadded to the nylon while it isv dissolved in the swelling agent, afterit is precipitated It Will be noted that in the' accompanyingdrawing,.the log ofthe compressive load is plotted against the re-` Ylciprocal of the temperature in degree absolute (degree` f centigrade-|273). Since two substantially straight linesVVV result from plottingincreasing sintering temperaturesl for l each material, it is apparentthat below the inilection'poin't 'f f on anycurve a singlel processisoperativel which/'is 'af function'solely of an activation energy and thetempera. 1

Above'the inflection point anew processis op- It is this gsecond ture'.erativey with Aa diiferent activation.

process with which this invention is concerned since sinte'r.VV p ing`below the inflection point is ineffective. Thisinilec'Y tion point isdisplayed notonly for theV pure 'polyamides but also for mixturescontaining polytetrauoroethylene@ p It isV not knownwhy the` strengthsuddenly begins'to increase at a rapid rate, but theinflection pointmay'bek easily determined for any given polyamide. "Reference f v,will'be made hereafter to the break in the curve'as 'the i.

The temperature to whichl the4V polyamide should be heated is thereforeabove the rinflection point and below the point where any substantial4molteninflection point.

phase occurs. If any substantial amount of molten' phase containing thepolymer of epsilon-caprolactam about C. to about 215 C. f

Before molding the powder mixture it may be ing is advantageouslyaccomplishedbyfplacing theigrDI-l:

granu lated, if desired, to obtain freer flowing powder; Mold-l latedpowder m'ntture in a mold or otherwise compressing it as by passing itthrough pressure rollers. p

After forming, the cold preformed nylon-polytetrauoroethylene article isthen sintered by heating it under nonoxidizing conditions. The timerange is usually between 2 to 30 minutes.

The presence of moisture in the nylon powder can, under some conditions,cause cracks to appear in the article on sintering. This is particularlytrue when the sintering is done in hot oil as contrasted with sinteringin vacuo. It has been 4found that relatively small bearings containing3% moisture or more before sintering, will crack if immersed directly inhot oil. On the other hand, a similar bearing first immersed in cold oiland then the oil raised slowly to the sintering temperature will be freefrom cracks. lt is therefore preferred to keep the moisture content oflthe formed nylon-polytetratluoroethylene article before sintering aslow as possible, preferably below about 1% moisture. In the case oflarge solid objects, the presence of moisture is more critical and avacuum drying step either on the powder mixture or the preformed articleis desirable.

In the above discussion the pressing has been assumed to take place atabout room temperature. If the temperature of the mold is raised to apoint close to the melting point of the nylon, very unsatisfactorymaterial is produced. A somewhat elevated temperature may be tolerated,however, and such temperature appears to add some strength to the greenarticle, but does not materially affect the finished product whensintered. It is essential therefore that the pressing be accomplished ata temperature below that at which any molten phase can occur andpreferably at or about room temperature.

The reason for the extremely tight bond obtained by my process is notapparent. It may be that the high pressure employed, followed bysintering, causes suthcient reorientation analogous to crystal growth tosecure adequate bonding. But whatever the reason may be, it is totallyunexpected that a strong bond would form and it is unexpected that anyamounts of polytetrauoroethylene up to 75 percent of the total weight orof polytetrafluoroethylene plus other filler up to 85 percent Vby bulkvolume would yield articles of surprising strength.

A further unexpected advantage is secured by my technique in that powderobtained from waste textile nylon, such as stocking material, may besuccessfully used either alone, or in conjunction with powder obtainedfrom virgin nylon. All attempts to melt and mold or extrude waste nylonhave resulted heretofore in excessively brittle products not suitablefor commercial use. It is apparent therefore that the high cost' factorsrestricting the wider use of molding nylon shapes have been largelyovercome by the process described herein. The special and expen-" sivetools required by the present techniques are replaced by the relativelysimple and available cold pressing equipment used for example in thepowder metallurgy art; and

the relatively expensive virgin nylon may be substituted in whole, or inpart, by nylon obtained from nylon scrap and textile waste, such astextile clippings and stockings.

If close tolerances are not required, the pressed, sin-p tered larticlesmay be satisfactorily used without subsequent conditioning or annealingto remove strains which may occur to a minor extent in the moldedarticles. In some bearings and other articles the necessity for closetolerances requires that the article =be free lfrom slight additionalshrinkages in service. Annealing is accomplished by simply heating thearticle, preferably under nonoxidizing conditions, to a temperatureunder, but

preferably close, i.e., within 50 C. to the melting point of thepolyamide for a few minutes to an hour and slowly cooling. Holding forlonger' times at lower temperatures also reduces strains. In some casesannealing may be `combined with the sinter'ing step to avoid two-furnace treatments. i

If desired, lubricating agents in minor amounts` -may be `added tofacilitate and improve uniformity of the' shaped piece and to aid theremoval of the pressed piece from the die or for other purposes. Usefullubricants include stearates such as zinc stearate, hydrogenated cottonseed oil, or other greasy or soapy substances which may be eitherintermixed with the polytetrafluoroethylene filled nylon powder orcoated on the die surfaces. Small amounts of other minor ingredients maybe added to improve the flowing properties of the dry powder mix-V turesor to aid in obtaining uniform physical properties in the sinteredproduct.

One of the important uses for pressed and sintered finely divided nyloncontaining polytetrauoroethylene has proved to be in the eld of bushingsand bearings. Polyhexamethylene adipamide in particular has exhibitedconsiderable merit for bushings or bearings operating where very lowcoeicients of friction are required. The bushings and bearings made inaccordance with this procfrom nylon by any of the present standardtechniques and have the advantages of beingfree from any gross` amountof strain and of possessing low coeflicients of friction. Tests haveindicated that the tendency to seize is less than that of injectionmolded bushings and bearings and therefore they may be made to closertolerances. Other articles which may be advantageously prepared by theprocess of this invention include various small irregular shapesemployed where the wear resistance and strength are important factors.Such articles include, but are not limited to, small rollers, cams,valve seats, gears, etc., and articles requiring good strength and lowcoeiiicients of frictions.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that I do not limit myself to the specific embodimentsthereof except as defined in the appended claims.

I claim:

1. A composition consisting essentially of a mixture of finely dividedlinear polyamide and polytetrailuoroethylene, said linear polyamidebeing a polyamide which is soluble in phenol at room temperature andinsoluble in ethylene glycol except at temperatures above about 140 C.and having an ultimate particle size of less than 40 microns, saidpolytetrailuoroethylene being present in a concentration of from 2 to 75percent by weight of said mixture. k

, 2. A composition consisting essentially of a mixture of finely dividedlinear polyamide and a filler, said linear polyamide being a polyamidewhich is soluble 1n phenol at room temperature and insoluble in ethylene,glycolv except atV temperatures above about 140 C.V and having anultimate particle size of less than 40 microns, said tiller beingpresent in a concentration up to percent of the bulk volume of themixture and containing polytetrauoroethylene in a concentration of from2 to 75 percent by weightof said polyamide-filler mixture.

3. A composition consisting essentially of a mixture of finely dividedlinear polyamide and polytetrauoroethy'lene, said linear polyamide beinga polyamide which rs soluble in phenol at room temperature and insolublein ethylene Vglycol except at temperatures above about C., said mixturehaving been formed by adding said polytetralluoroethylene to a solutionof said polyamide in ethylene glycol, cooling to co-precipitate theresulting mixture of polyamide and polytetrauoroethylene, and washingand drying said co-precipitated mixture, said polytetrauoroethylenebeing present in a concentration of `from 2 toabout 75 percent by weightof said mixture.

4. A shaped polyamide article consisting essentially of sinteredpolyhexamethylene adiparnide' and polytetrafluor'oethylene, saidpolyhexamethylene adipainide having an average ultimate particle size ofless than 40 microns, saidpolytetrauoroethylene constituting from about40 to 50 percent by weight of said article, said article beingcharacterized by being bonded by sintering action alone.

5. A shaped polyamide article consisting essentially of sinteredpoly-hexamethylene sebacamide and polytetrauoroethylene, saidpolyhexamethylene sebacamide having an average ultimate particle size ofless than 40 microns, said polytetnafluoroethylene constituting fromabout 20 to 30 percent by weight of said article, said article beingcharacterized by being bonded by sintering action alone.

6. A shaped polyamide article consisting essentially of sinteredepsilon-caprolactam polymer and polytetrauoroethylene, saidepsilon-caprolactam polymer having an average ultimate particle size ofless lthan 40 microns, said polytetrafluoroethylene constituting fromabout 3 to percent by weight of said article, said article beingcharacterized by being bonded by sintering action alone.

7. A bearing of low coecient of friction consisting essentially ofsintered polyhexamethylene adipamide and polytetrauoroethylene, saidpolyhexamethylene adipamide having an average ultimate particle size ofless than 40 microns, said polytetrauoroethylene constituting about 50percent by weight of said bearing, said bearing being characterized bybeing bonded by sintering action alone.

8. A shaped polyamide article of low coeicient of friction consistingessentially of sintered synthetic linear polyamide andpolytetrafluoroethylene, said polyamide being one which is soluble inphenol at room temperature and insoluble in ethylene glycol except attemperatures above about 140 C. and having an ultimate particle size ofless than 40 microns, said article containing 10 a suicient amount ofpolytetrauoroethylene to materially reduce the coeicient of friction ascompared with that of the polyamide alone but not more than about 75percent by weight of said article, said article being characterized bybeing bonded by sintering action alone.

9. A composition for use in -forming articles of low coefficient offriction consisting essentially of a mixture of finely divided linearpolyamide and polytetrauoroethylene, said linear polyamide being apolyamide which is soluble in phenol 'at room temperature and insolublein ethylene glycol except at temperatures `above about 140 C. and havingan ultimate particle size of less than 40 microns, the compositioncontaining a suicient amount of the polytetrailuoroethylene tomaterially reduce the coefficient of friction of an Aarticle formed fromthe composition -as compared to an article made from the polyamidealone, but not more than about percent by weight of said mixture.

References Cited in the le of this patent UNITED STATES PATENTS2,246,086 Austin June 17, 1941 2,345,533 Graves Mar. 28, 1944 2,400,091Alfthan May 14, 1946 2,592,616 Stott et al. Apr. 15, 1952 2,639,278Stott et al. May 19, 1953 2,695,425 Stott Nov. 30, 1954 2,698,966 Stottet al. Jan. 11, 1955 2,748,099 Bruner et al. May 29, 1956

1. A COMPOSITION CONSISTING ESSENTIALLY TO A MIXTURE OF FINELY DIVIDEDLINEAR POLYAMIDE AND POLYTETRAFLUOROETHYLENE, SAID LINEAR POLYAMIDEBEING A POLYAMIDE WHICH IS SOLUBLE IN PHENOL AT ROOM TEMPERATURE ANDINSOLUBLE IN ETHYLENE GLYCOL EXCEPT AT TEMPERATURES ABOVE ABOUT 140* C.AND HAVING AN ULTIMATE PARTICLE SIZE OF LESS THAN 40 MICRONS, SAIDPOLYTETRAFLUOROETHYLENE BEING PRESENT IN A CONCENTRATION OF FROM 2 TO 75PERCENT BY WEIGHT OF SAID MIXTURE.